Electric vehicles are receiving considerable attention as a substitute for present gasoline-fueled vehicles. This interest is based primarily on zero atmospheric emissions obtainable from an all-electric vehicle. Several states are considering stricter emissions regulations for vehicles, and California has adopted regulations that will go into effect and require zero emissions for a percentage of vehicles when operating in certain urban areas. Electric vehicles also offer other advantages including reducing dependency on imported oil, since utilities in the United States generate a large portion of their energy demands using coal, gas, nuclear, and hydroelectric energy sources.
A typical electric vehicle has a power train including a rechargeable traction battery powering one or more electric motors through a suitable power drive circuit. The traction battery typically includes a plurality of rechargeable battery cells for powering the vehicle. For example, U.S. Pat. No. 4,449,080 entitled Electric Vehicle Protection Scheme to Konrad et al. discloses an electric vehicle including a traction battery, a power control circuit, and a DC drive motor. The patent also discloses a main contactor including a pair of contacts connected in series with the traction battery and the power control circuit. The contactor permits the traction battery to be connected during start-up and motoring, and disconnected otherwise. In addition, the main contactor also operates in combination with a circuit fault detector to disconnect the traction battery if a fault is detected in either of a chopper circuit or a chopper bypass contactor.
While DC motors have been used in electric vehicles, there has been a reduction in the cost inverters for changing DC into AC. Accordingly, AC drive trains are now advantageously used in electric vehicles. An AC drive train offers advantages in terms of power-to-weight ratio and improved overall vehicle operating efficiency. The voltage of the traction battery is desirably relatively high, such as above 300 volts, also to provide greater efficiency. Unfortunately, this relatively high battery voltage may readily damage contacts of a contactor for connecting and disconnecting the traction battery from the DC-to-AC inverter.
In general, a larger air gap is required for contacts to interrupt current with increasing voltage. Moreover, with a DC supply, such as a typical traction battery, it is often necessary with voltages above 100 volts to incorporate a blow out coil or arc cooler to ensure that the arc will be extinguished when the contacts are opened. Such additional components add to the expense and complexity of the drive train, as well as increase the vehicle weight.
A main contactor for an electric vehicle having an AC drive is typically not intended to serve as the principle means of interruption for a circuit fault; rather, fault interruption is provided by fusing. The main contactor thus serves primarily for isolating the traction battery from the other drive train components when the vehicle is deenergized, and for connecting the battery to the power train when the vehicle is being driven. Accordingly, the function of the contactors is to make and break the circuit to the traction battery while only minimal current is flowing, and to provide a conductive path to or from the battery when the vehicle is motoring or regeneratively braking, respectively.
Typically the input to a DC-to-AC inverter includes a large filter capacitance, for example, of about 20,000 mfd. During normal operation, this capacitance is charged to a voltage essentially equal to that of the traction battery so that when the contacts are opened, no significant voltage appears across the contacts. Unfortunately, if a contactor is inadvertently opened during motoring, either due to a software error, a loose connection or other reason, the charge on the capacitive input filter is quickly depleted in a few milliseconds and the contacts are thus forced to interrupt a current with essentially full traction battery voltage impressed across the contacts. This may lead to an early failure of the contacts. In addition, other circuit components may be damaged by a high current flow resulting from the contacts being open in error.
Another problem associated with contact damage is the inadvertent closure of the contacts when the input filter capacitor of the DC-to-AC inverter is not charged to a voltage nearly equal to that of the traction battery. If the contactor is closed when the capacitor is not sufficiently charged, welding of the contacts may occur. When the contacts first touch, an immediate inrush current of substantial magnitude will flow through the contacts. In addition, it is not uncommon for the contacts to bounce open and closed several times for a few milliseconds during closure. With the high inrush current, arcing occurs which leads to localized heating and melting of tip material. When the bouncing subsides, the molten material of the contacts cools and welds the contacts together.